The present exemplary embodiment relates to a fuser apparatus for an electrophotographic marking device and, more particularly, to control of an operating temperature of a fuser apparatus.
In typical xerographic image forming devices, such as copy machines and laser beam printers, a photoconductive insulating member is charged to a uniform potential and thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member, which corresponds to the image areas contained within the document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with a marking material. Generally, the marking material comprises pigmented toner particles adhering triboelectrically to carrier granules, which is often referred to simply as toner. The developed image is subsequently transferred to print medium, such as a sheet of paper.
The fusing of the toner image onto paper is generally accomplished by applying heat and pressure. A typical fuser assembly includes a fuser roll and a pressure roll which define a nip therebetween. The side of the paper having the toner image typically faces the fuser roll, which is often supplied with an internal heat source, such as a resistance heater, e.g., a lamp, in its interior. The combination of heat from the fuser roll and pressure between the fuser roll and the pressure roll fuses the toner image to the paper, and once the fused toner cools, the image is permanently fixed to the paper
The paper passing through the fuser absorbs heat from the fuser roll. The temperature of the roll is measured by a thermistor and power is supplied to the resistance heater to maintain the fuser roll at a desired operating temperature.
Because the paper passing through the nip absorbs heat from the fuser roll, once a print job has ended and the cooling effect of the paper is no longer present, the temperature fuser roll surface tends to rise, due to the thermal gradient within the fuser roll. Accordingly, the printer is often cycled into a non-operational mode for a period of time to allow the fuser roll to reach its operating temperature. After one printing job is done, the next job has to wait until each fuser member gets back to its temperature set range. This inter-cycle time depends on the fuser system as well as media type and previous job length. Since the fuser roll has a large thermal inertia, it is usually the last roll to get ready for the next job. For example, in a fuser which has been operating at a surface temperature of 185° C. while printing a coated thick paper, the surface temperature may stay above 185° C. for several minutes as there is no active cooling on the fuser surface. Additionally, in a nip-forming fuser assembly, the fuser roll surface may reach a much higher temperature than is desirable for the fuser surface, leading to premature degradation of the rubber or other compressible material forming the fuser roll surface.
One proposal for reducing these effects is to use the pressure roll to cool off the fuser roll surface. However, this can lead to undesired oil transfer to the pressure roll. Another option is to blow compressed air on the fuser roll surface or through the roll cavity. However, it is difficult to cool the fuser roll evenly by this method. As a result, thermal streaking may occur. Additionally, the exhaustion of the hot air is a concern.
There remains a need for a method for controlling the thermal gradient in a fuser roll.